1. Field of the Invention
The present invention generally relates to display control apparatuses, and more particularly to a display control apparatus in which images are displayed on a screen thereof under a control of a central processing unit (CPU).
2. Prior Art
Recently, a display apparatus shown in FIG. 1 is used as a terminal unit of the so-called captain system concerning a new media system. In FIG. 1, 1 designates a central processing unit (CPU), 2 designates a memory constituted by a read only memory (ROM) and a random access memory (RAM). The ROM stores programs used in the CPU 1, and the RAM stores data. In addition, 3 designates a display control apparatus, 4 designates a video random access memory (VRAM), 7 designates a CRT display unit, and 8 designates a video signal generating source such as a video tape recorder (VTR) or a television camera or the like. The VRAM 4 stores color codes each corresponding to each display dot on a screen of the CRT display unit 7. These color codes are read from the VRAM 4 in parallel based on address data ADS, and read color codes are supplied to the display control apparatus 3. On the other hand, the display control apparatus 3 writes the color codes supplied from the CPU 1 into the VRAM 4. When the CPU 1 outputs a display command to the display control apparatus 3, the display control apparatus 3 outputs the address data AD to the VRAM 4 so as to sequentially read the color codes from the VRAM 4 in parallel. Next, the display control apparatus 3 converts the color codes into analog signals, such as a red signal R, a green signal G and a blue signal B. The display control apparatus outputs these analog signals with a synchronizing signal SYNC to the CRT display unit 7. Thus, a color dot pattern (hereinafter, referred to as a VRAM image) corresponding to the color codes stored in the VRAM 4 is displayed on the screen of the CRT display unit 7.
In the case where the color code read from the VRAM 4 represent a transparent color, the display control apparatus 3 outputs a signal YS to a YS terminal (not shown) of the CRT display unit 7. At this time, the CRT display unit 7 displays an image (hereinafter, referred to as an external image) corresponding to a composite color video signal from the external video signal generating source 8, instead of the VRAM image. As shown in FIG. 2A, an external image G2 is inserted into a partial area of a VRAM image G1 (corresponding to an area within the VRAM 4 in which transparent color codes are written), for example. Or, as shown in FIG. 2B, the VRAM image G1 is inserted into a partial area of the external image G2.
Meanwhile, the conventional display apparatus uses an image compounding technique such as a so-called chromakey in a television broadcasting industries. Next, description will be given with respect to a basic principle of the chromakey. First, a first television camera picks up an image of a man M who stands up in front of a blue background B, as shown in FIG. 3(a). The blue color is a complementary color of a skin color of the man M and a hue difference between the blue color and the skin color is quite large, hence, the color of the background B is determined to the blue color. Next, a circuit called "chromakeyer" (not shown) detects a difference between color signals corresponding to respective colors of the background B and the man M. This circuit drives a gate signal generator (not shown). Based on a first gate signal outputted from the gate signal generator, a signal corresponding to the background B is removed from an image signal outputted from the first television camera so that a first image signal only corresponding to the man M can be obtained. Next, a second television camera picks up an image of a background H only, as shown in FIG. 3(b). Based on a second gate signal inverse to the first gate signal, a signal corresponding to the man M is removed from the an image signal outputted from the second television camera so as to obtain a second image signal corresponding to an image in which an area of the man M is extracted from the background H.
The above first and second image signals are compounded together in a compounding amplifier (not shown), whereby a compounded image G (shown in FIG. 3(c) can be obtained.
Recently, the display apparatus is required to provide several kinds of specific display functions. One of these specific display functions is the chromakey function described above. Even the conventional display apparatus (shown in FIG. 1) can obtain an effect similar to the chromakey effect. More specifically, as shown in FIG. 2B, an animation image of the man (i.e., the VRAM image G1) can be inserted into the background image (i.e., the external image G2). In this case, only the transparent color codes can determine an extracting area of the VRAM image G1. Hence, the conventional display apparatus suffers a problem in that the chromakey function must be limited by large extent because the extracting area can not be determined by the color codes representing colors other than the transparent color. In addition, the VRAM image G1 stored in the VRAM 4 must be always identified as a reference image when the images are compounded. Hence, it is impossible for the conventional display apparatus to extract a red color portion from the external image G2 and insert such red color portion into the VRAM image G1. In this case, it can be easily thought out that the composite video signal is converted into a digital signal and an operational process is performed on the digital signal so as to extract the required portion (such as the above red color portion). However, when the above-mentioned process is performed by use of a software, the conventional display apparatus suffers another problem in that a processing time of the CPU 1 must become longer.